This invention relates to a nitrile group-containing highly saturated copolymer rubber giving a crosslinked rubber product having excellent cold resistance, a crosslinkable rubber composition comprising the highly saturated copolymer rubber, and a crosslinked rubber product made by crosslinking the rubber composition.
In recent years, a nitrile group-containing highly saturated copolymer rubber represented by a hydrogenated acrylonitrile-butadiene copolymer rubber attracts attention. A nitrile group-containing highly saturated copolymer rubber has a reduced amount of carbon-carbon unsaturated bonds in the backbone chain as compared with an acrylonitrile-butadiene copolymer rubber, and thus, the highly saturated copolymer rubber has good heat resistance, oil resistance and ozone resistance.
However, the cold resistance of a highly saturated nitrile group-containing is occasionally reduced depending upon the content of a nitrite group and the content of carbon-carbon double bonds in the nitrile group-containing highly saturated copolymer rubber.
In general, cold resistance can be enhanced by reducing the content of an acrylonitrile group in an acrylonitrile-butadiene copolymer rubber. But, cold resistance of a nitrile group-containing highly saturated copolymer rubber is occasionally increased and occasionally not increased, when the content of a nitrile group is reduced.
To improve cold resistance of a nitrile group-containing highly saturated copolymer rubber, a nitrile group-containing highly saturated copolymer rubber comprising four kinds of monomer units. i.e. (a) xcex1,xcex2-ethylenically unsaturated nitrile monomer units, (b) xcex1,xcex2-ethylenically unsaturated carboxylic acid ester monomer units, (c) conjugated diene monomer units, and (d) saturated conjugated diene monomer units, was proposed in, for example, Japanese Unexamined Patent Publication No. S63-95242 and ibid. H3-109449. However, the proposed nitrile group-containing highly saturated copolymer rubber gives a crosslinked rubber product which is liable to occasionally exhibits insufficient cold resistance and physical properties varying when it is placed in contact with oil.
A primary object of the present invention is to provide a nitrile group-containing highly saturated copolymer rubber giving a crosslinked rubber product having good cold resistance, oil resistance and dynamic properties; a crosslinkable rubber composition comprising the highly saturated copolymer rubber; and a crosslinked rubber product made by crosslinking the rubber composition.
To achieve the above-mentioned object, the present inventors made extensive research and found that a nitrile group-containing highly saturated copolymer rubber having a specific monomer unit composition, and exhibiting a reduced temperature difference between the extrapolated glass transition-initiating temperature (Tig) and the extrapolated glass transition-ending temperature (Teg), as measured by the differential scanning calorimetry, which copolymer rubber is prepared by carrying out polymerization while the monomer concentration In a polymerization reaction mixture is controlled depending upon the reactivity of monomers, gives a crosslinked rubber product having good cold resistance, oil resistance and dynamic properties. Based on this finding, the present invention has been completed.
Thus, in one aspect of the present Invention, there is provided a nitrile group-containing highly saturated copolymer rubber comprising (a) 10 to 40% by weight of xcex1,xcex2-ethylenically unsaturated nitrile monomer units, (b) 10 to 60% by weight of xcex1,xcex2-ethylenically unsaturated carboxylic acid ester monomer units, (c) 0.01 to 21% by weight of conjugated diene monomer units, and (d) 14 to 69. 99 by weight of saturated conjugated diene monomer units, wherein the sum of monomer units (c) and monomer units (d) is in the range of 20 to 70% by weight, the ratio of monomer units (d) to the sum of monomer units (c) and monomer units (d) is at least 70% by weight, and the difference between the extrapolated glass transition-initiating temperature (Tig) and the extrapolated glass transition-ending temperature (Teg), as measured by the differential scanning calorimetry, is not higher than 10xc2x0 C.
In another aspect of the present invention, there is provided a crosslinkable rubber composition comprising 100 parts by weight of the above-mentioned nitrile group-containing highly saturated copolymer rubber, and 0.1 to 5 parts by weight of a sulfur-containing vulcanizing agent or 1 to 16 parts by weight of an organic peroxide crosslinking agent.
In still another aspect of the present invention, there is provided a crosslinked rubber product made by crosslinking the above-mentioned crosslinkable rubber composition.
(Nitrile Group-Containing Highly Saturated Copolymer Rubber)
The nitrile group-containing highly saturated copolymer rubber of the present invention rubber comprises (a) 10 to 40% by weight of xcex1,xcex2-ethylenically unsaturated nitrile monomer units, (b) 10 to 60% by weight of xcex1,xcex2-ethylenically unsaturated carboxylic acid ester monomer units, (c) 0.01 to 21% by weight of conjugated diene monomer units, and (d) 14 to 69.99% by weight of saturated conjugated diene monomer units, wherein the sum of monomer units (a) and monomer units (d) is in the range of 20 to 70% by weight, the ratio of monomer units (e) to the sum of monomer units (c) and monomer units (d) is at least 70% by weight, and the difference between the extrapolated glass transition-initiating temperature (Tig) and the extrapolated glass transition-ending temperature (Teg), as measured by the differential scanning calorimetry, is not higher than 10xc2x0 C.
As specific examples of an xcex1,xcex2-ethylenically unsaturated nitrile monomer forming the xcex1,xcex2-ethylenically unsaturated nitrile monomer units (a), there can be mentioned acrylonitrile; xcex1-halogenoacrylonitrile such as xcex1-chloroacrylonitrile and xcex1-bromoacrylonitrile; and xcex1-alkylacrylonitrile such as mehacrylonitrile and ethacrylonitrile. Of these, acrylonitrile is preferable. The xcex1,xcex2-ethylenically unsaturated nitrile monomers may be used either alone or as a combination of at least two thereof.
The content of xcex1,xcex2-ethylenically unsaturated nitrile monomer units (a) in the nitrile group-containing highly saturated copolymer rubber is in the range of 10 to 40% by weight, preferably 12 to 35% by weight and more preferably 15 to 30% by weight. When the content of xcex1,xcex2-ethylenically unsaturated nitrile monomer units (a) is too small, the resulting crosslinked rubber product has reduced oil resistance. In contrast, when the content of xcex1,xcex2-ethylenioally unsaturated nitrile monomer units (a) is too large, the resulting crosslinked rubber product has poor rubber cold resistance.
As specific examples of an xcex1,xcex2-ethylenically unsaturated carboxylic acid eater monomer forming the xcex1,xcex2-ethylenically unsaturated carboxylic acid ester monomer units (b), there can be mentioned acrylates and methacrylates, which have an alkyl group having 1 to 18 carbon atoms, such as methyl acrylate, ethyl acrylate, butyl acrylate, n-dodecyl acrylate, methyl methacrylate and ethyl methacrylate; acrylates and methacrylates, which have an alkoxyalkyl group having 2 to 18 carbon atoms, such as methoxymethyl acrylate and methoxyethyl methacrylate; acrylates and methacrylates, which have a cyanoalkyl group having 2 to 18 carbon atoms, such as xcex1-cyanoethyl acrylate, xcex2-cyanoethyl acrylate and cyanobutyl acrylate, acrylates and methacrylates, which have a hydroxyalkyl group having 1 to 18 carbon atoms, such as 2-hydroxyethyl acryalte, hydroxypropyl acrylate and 2-hydroxyethyl methacrylate; acrylates and methacrylates, which have an aminoalkyl group with alkyl groups each having 1 to 18 carbon atoms, such as dimethylaminomethyl acrylate, diethylaminoethyl acrylate and dimethylaminoethyl methacrylate; acrylates and methacrylates, which have a trifluoroalkyl group having 1 to 18 carbon atoms, such as trifluoroethyl acrylate and tetrafluoropropyl methacrylate; benzyl acrylates having a fluoro-substituent and benzyl methacrylates having a fluoro-substituent, such as fluorobenzyl acrylate and fluorobenzyl methacrylate; and unsaturated dicarboxylic acid monoalkyl esters and unsaturated dicarboxylic acid dialkyl esters, which have an alkyl group or alkyl groups having 1 to 18 carbon atoms, such as monoethyl maleate, dimethyl maleate, dimethyl fumarate, dimethyl itaconate, n-butyl itaconate and diethyl itaconate. Of these, acrylates and methacrylates, which have an alkyl group, are preferable. Butyl acrylate is especially preferable. The xcex1,xcex2-ethylenioally unsaturated carboxylic acid ester monomers may be used either alone or as a combination of at least two thereof.
The content of xcex1,xcex2-ethylenically unsaturated carboxylic acid ester monomer units (b) in the nitrile group-containing highly saturated copolymer rubber is In the range of lo to 60% by weight, preferably 15 to 55% by weight and more preferably 20 to 50% by weight. When the content of xcex1,xcex2-ethylenically unsaturated carboxylic acid ester monomer units (b) is too small, the resulting crosslinked rubber product has poor cold resistance. In contrast, when the content of xcex1,xcex2-ethylenically unsaturated carboxylic acid monomer units (b) is too large, the resulting crosslinked rubber product has poor oil resistance and dynamic properties.
As specific examples of conjugated diene monomer forming conjugated diene monomer units (c) in the nitrile group-containing highly saturated copolymer rubber, there can be mentioned 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and 1,3-pentadiene, Of these, 1,3-butadiene is preferable. The conjugated diene monomers may be used either alone or as a combination of at least two thereof.
The content of conjugated diene units (c) in the nitrile group-containing highly saturated copolymer rubber is in the range of 0.01 to 21% by weight, preferably 0.05 to 16.25% by weight and more preferably 0.01 to 21% by weight. When the content of conjugated diene units (c) is too large, the resulting crosslinked rubber product has poor heat resistance. In contrast, when the content of conjugated diene units (c) is too small, the rubber product has reduced crosslinkability, and a crosslinked rubber product has poor mechanical strength, even if the crosslinked rubber product can be produced.
The saturated conjugated diene units (d) in the nitrile group-containing highly saturated copolymer rubber have a structure such that the carbon-carbon double bonds of conjugated dione monomer units have been at least partly saturated by hydrogenation.
The content of saturated conjugated diene monomer units (d) in the nitrite group-containing highly saturated copolymer rubber is in the range of 14 to 69.99% by weight, preferably 18.75 to 64.95% by weight and more preferably 28 to 59.9% by weight. When the content of saturated conjugated diene monomer units (d) is too small, the crosslinked rubber product has poor heat resistance. In contrast, when the content of saturated conjugated diene monomer units (d) is too large, the crosslinked rubber product exhibits poor dynamic properties and undesirably large compression set.
The sum of conjugated diene monomer units (c) and saturated conjugated diene monomer units (d) in the nitrile group-containing highly saturated copolymer rubber of the present invention is in the range of 20 to 70% by weight, preferably 25 to 65% by weight and more preferably 35 to 60% by weight. When the sum of conjugated diene monomer units (c) and saturated conjugated diene monomer units (d) is too small, the resulting crosslinked rubber product has poor dynamic properties. In contrast, when the sum of conjugated diene monomer units (c) and saturated conjugated diene monomer units (d) is too large, the resulting crosslinked rubber product has poor cold resistance and oil resistance.
The ratio of monomer units (d) to the sum of monomer units (c) and monomer units (d) is at least 70% by weight, preferably at least 75% by weight and more preferably at least 80% by weight. If this ratio is too small, the resulting crosslinked rubber product has poor heat resistance, oil resistance and ozone resistance, preferable.
The nitrile group-containing highly saturated copolymer rubber of the present invention preferably has a number average a molecular weight in the range of 10,000 to 2,000,000, more preferably 30,000 to 1,500,000 and especially preferably 50,000 to 1,000,000, When the number average molecular weight is too small, the rubber tends to have too low viscosity and have poor mechanical strength such as tensile strength. In contrast, when the number average molecular weight Is too large, the rubber tends to have too high viscosity and have poor processability.
In the nitrile group-containing highly saturated copolymer rubber of the present invention, the temperature difference (xcex94T) between the extrapolated glass transition-initiating temperature (Tig) and the extrapolated glass transition-ending temperature (Teg), as measured by the differential scanning calorimetry according to JIS K7121 xe2x80x9cmethod of measuring transition temperature of plasticsxe2x80x9d, is not higher than 10xc2x0 C., preferably not higher than 9xc2x0 C., and more preferably not higher than 8.5xc2x0 C.
In the nitrile group-containing highly saturated copolymer rubber of the present invention, the monomer units (a), monomer units (b), and the sum of monomer units (c) plus monomer units (d) preferably have a composition distribution breadth of not larger than 20%, more preferably not larger than 15% by weight and especially preferably not larger then 10% by weight, When the composition distribution breadth is too large, the temperature difference (xcex94T) between the extrapolated glass transition-initiating temperature (Tig) and the extrapolated glass transition-ending temperature (Teg) is liable to become undesirably large.
By the term xe2x80x9ccomposition distribution breadthxe2x80x9d as herein used, we mean the value in % as defined by the equation:
[(Mmaxxe2x88x92Mmin)/Mt]xc3x97100
wherein M is content (%) of units of a certain monomer in the total polymer, Mmax is the maximum content (%) of units of the monomer as measured on minute sections of the polymer, and Mmin is the minimum content (%) of units of the monomer as measured on the minute sections of the polymer. More specifically content Ms (%) of units of a monomer is measured on minute sections each having a length corresponding to 1 to 5% by weight, preferably 2 to 4% by weight, of a polymer molecule as calculated on the basis of number average molecular weight. The maximum value of content Ms and the minimum value of content Ms are Mmax and Mmin, respectively, as measured on the respective minute sections of polymer. The content Ms of units of a monomer in each minute section of polymer can be determined by measuring the amount of monomer consumed per each stage during which the polymerization conversion is increased by a predetermined value.
The content of each of monomer units (a), monomer units (b), monomer units (c) and monomer units (d) in the nitrile group-containing copolymer rubber can be determined advantageously by employing a combination of plural methods selected from nitrogen content-determination by semi-micro Kjeldahl method, unsaturation content-determination by infrared absorption spectroscopy or iodine value determination, and identification of partial structures or content ratio determination by infrared absorption spectroscopy, 1H-NMR, 13C-NMR and pyrolysis gas chromatography. Of these, identification of partial structures or content ratio determination by 1H-NMR is generally most reliable, but, a plurality of peaks in a 1H-NMR chart occasionally coincide with each other which render the determination difficult. Therefore, a combination of 1H-NMR with other methods is especially preferable.
The nitrile group-containing highly saturated copolymer rubber of the present invention is preferably made by a process wherein xcex1,xcex2-ethylenically unsaturated nitrile monomer, xcex1,xcex2-ethylenically unsaturated carboxylic acid ester monomer and conjugated dione monomer are copolymerized, and then the conjugated diene monomer units in the copolymer are selectively hydrogenated. Monomer concentrations in a polymerization reaction mixture are controlled by adding monomers depending upon the reactivity of monomers in the course of copolymerization so that each monomer composition breadth of monomer units (a), monomer units (b) and monomer units (c) becomes small. For example, a target molecular weight of a polymer is predetermined, and the concentrations of monomers in each section corresponding to 1 to 5% by weight, preferably 2 to 4% by weight, of the total polymerization conversion were controlled. The control can be effected by adding monomers in the course of copolymerization, The amounts of monomers added are varied depending upon the reactivity of monomers. It is not necessary to measure the monomer concentrations in each section. Namely, a manner in which the concentration of each monomer should be controlled in the course of polymerization is determined by a preliminary experiment, and the polymerization can be carried out according to the previously determined manner. A predominant part of the preliminary experiment can be achieved by simulation by a computer and the results obtained by simulation can be confirmed by experiment.
Hydrogenation of the copolymer rubber comprising monomer units (a), monomer units (b) and monomer units (c) is carried out to an extent such that the ratio of monomer units (d) to the sum of monomer units (c) plus monomer units (d) reaches at least 70% by weight, preferably at least 75% by weight and more preferably at least 80% by weight. The copolymer rubber before the hydrogenation does not comprise monomer units (d), but comprises monomer units (c). The composition distribution breadth of monomer units (c) in the copolymer rubber before hydrogenation is substantially the same as that of the sum of monomer units (c) plus monomer units (d) in the copolymer rubber after hydrogenation.
Other polymerization conditions, for example, a polymerization medium, concentration of polymerization reaction liquid, kind and amount of a polymerization initiator, a polymerization temperature and a polymerization conversion at termination of polymerization, and hydrogenation conditions such as kind and amount of a hydrogenation catalyst and a hydrogenation temperature can be appropriately chosen according to the conventional procedures for producing a nitrile group-containing copolymer rubber and hydrogenating the nitrile group-containing copolymer rubber to produce a nitrile group-containing highly saturated copolymer rubber.
(Crosslikable Rubber Composition)
The crosslinkable rubber composition of the present invention comprises as essential ingredients the above-mentioned nitrile group-containing highly saturated copolymer rubber and a crosslinking agent, and other ingredients as optional Ingredients.
The crosslinking agent used is not particularly limited provided that it is capable of crosslinking the nitrile group-containing highly saturated copolymer rubber of the present invention. However, a sulfur-containing crosslinking agent and an organic peroxide crosslinking agent are preferably used.
As specific examples of the sulfur-containing crosslinking agent, there can be mentioned sulfur such as powdery sulfur and precipitated sulfur; and organic sulfur compounds such as 4,4xe2x80x2-dithiomorpholine, tetramethylthiuram disulfide, tetraethylthiuram disulfide and high molecular weight polysulfide. The amount of the sulfur-containing crosslinking agent is in the range of 0.1 to 5 parts by weight, preferably 0.2 to 4.5 parts by weight and more preferably 0.3 to 4 parts by weight, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber. When the amount of the sulfur-containing crosslinking agent is too small, the crosslinking density of rubber is reduced and the compression set becomes large. In contrast, when the amount of the sulfur-containing crosslinking agent Is too large, the resulting crosslinked rubber is liable to have poor flexural fatigue resistance and high dynamic heat build-up.
When a sulfur-containing crosslinking agent is used, a crosslinking accelerator such as zinc oxide, guanidine crosslinking accelerators, thiazole crosslinking accelerators, thiuram crosslinking accelerators or dithiocarbamate crosslinking accelerators is preferably used.
The organic peroxide crosslinking agent includes those which are used in rubber industry, such as dialkyl peroxides, diacyl peroxides and peroxyesters. Of these, dialkyl peroxides are preferable. As specific examples of the organic peroxide crosslinking agent, there can be mentioned dialkyl peroxides such as dicumyl peroxide, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di(tert-butyl-peroxy)-3-hexyne, 2,5-dimethyl-2,5-di(tert-butyl-peroxy)hexane and 1,3-bis(tert-butyl-peroxyisopropyl)benzene; diacyl peroxides such as benzoyl peroxide and isobutyryl peroxide; and peroxy esters such as 2,5-dimethyl-2,5-bis(benzoyl-peroxy)hexane and tert-butyl-peroxyisopropyl carbonate, The amount of the organic peroxide crosslinking agent is in the range of 1 to 16 parts by weight, preferably 1 to 14 parts by weight and more preferably 1 to 12 parts by weight, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber. When the amount of the organic peroxide crosslinking agent is too small, the crosslinking density of rubber is reduced and the compression set becomes large. In contrast, when the amount of the organic peroxide crosslinking agent is too large, the resulting crosslinked rubber is liable to have poor rubber elasticity.
When an organic peroxide crosslinking agent is used, a crosslinking accelerator such as triallyl isocyanurate, trimethylolpropane trimethacrylate or N,Nxe2x80x2-m-phenylenedimalerimide is preferably used.
The crosslinking accelerators may be used either alone or as a combination of at least two thereof. The crosslinking accelerator can be used as a dispersion in clay, calcium carbonate or silica whereby processability is enhanced. The amount of crosslinking accelerator is not particularly limited, and may be appropriately chosen depending upon the use of and properties required for crosslinked rubber product, the kind of crosslinking agent and the kind of crosslinking accelerator.
According to the need, various ingredients can be incorporated, in addition to the nitrile group-containing highly saturated copolymer rubber, the crosslinking agent and optional crosslinking accelerator and accelerator activator, in the rubber composition of the present invention. The ingredients include those which are conventionally used in rubber industry, for example, a reinforcing tiller such as carbon black or silica, a non-reinforcing filler such as calcium carbonate or clay, a processing aid, a plasticizer, an antioxidant, an anti-ozonant and a colorant. The amount of these ingredients is not particularly limited provided that the object and effect of the present invention can be achieved, and suitable amounts can appropriately chosen depending upon the particular use of ingredients.
Various rubbers, other than the nitrile group-containing highly saturated copolymer rubber, can be incorporated. The rubbers used are not particularly limited, But, when a nitrile group-containing copolymer rubber having a high degree of unsaturation such as the conventional acrylonitrile-butadiene copolymer rubber is incorporated. its amount should be not larger than 30 parts by weight, preferably not larger than 20 parts by weight and more preferably not larger than 10 parts by weight, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber. When the amount of a nitrile group-containing copolymer rubber having a high degree of unsaturation is too large, a crosslinked rubber product having good hot-air aging resistance, flexural fatigue resistance and elongation and reduced compression set cannot be obtained. When rubbers other than the nitrile group-containing highly saturated copolymer rubber are additionally used, a crosslinking agent capable of crosslinking these rubbers can be additionally used.
The procedure for preparing the crosslinkable rubber composition of the present invention is not particularly limited, and the rubber composition can be prepared by conventional procedures employed for general rubber compositions. An appropriate mixing method using a closed type mixer or a roll mixer can be employed, When a crosslinking agent is incorporated in the rubber composition, the kneading after incorporation of the crosslinking agent should be carried out at a temperature lower than the crosslinking-initiating temperature so as to avoid premature crosslinking. Usually, ingredients which are not easily thermally decomposed are first incorporated with the rubber, and then, crosslinking agent and crosslinking accelerator are added at a temperature lower than the crosslinking-initiating temperature.
(Crosslinked Rubber Product)
The crosslinked rubber product of the present invention is made by crosslinking the above-mentioned crosslinkable rubber composition. The method for making the crosslinked rubber product is not particularly limited. Usually the crosslinkable rubber composition is heated to effect crosslinking.
The heating temperature at crosslinking is preferably in the range of 100 to 200xc2x0 C., more preferably 130 to 200xc2x0 C. and especially preferably 140 to 200xc2x0 C. When the heating temperature is too low, a substantially long time is required for crosslinking and the crosslinking density is liable to be reduced. In contrast, when the heating temperature is too high, the crosslinking time is too short and a defective molding is liable to be produced.
The crosslinking time can be appropriately chosen depending on the crosslinking method, crosslinking temperature and shape of the rubber product, and usually in the range of one minute to 20 hours in view of the crosslinking density and the production efficiency.
The heating means may be appropriately chosen from those which are employed for crosslinking rubbers and which includes, for example, press-heating, steam-heating, oven-heating and hot air-heating.
Now the invention will be specifically described by the following examples and comparative examples wherein parts and % are by weight unless otherwise specified. The properties of rubber were evaluated by the following methods.
(1) Dry Physical Properties
A crosslinkable rubber composition was press-cured at a temperature of 160xc2x0 C. under a pressure of 10 MPa for 20 minutes, and then subjected to second curing at temperature of 150xc2x0 C. for 2 hours by using a Geer oven to prepare a sheet with a thickness of 2 mm. The sheet was die-cut by a #3 dumbbell die to prepare a specimen. Breaking tensile strength, tensile modulus at 100% elongation and breaking elongation were measured according to Japanese Industrial Standard (JIS) K6251. Hardness was measured by using a durometer hardness tester, type A, according to JIS K6253.
(2) Hot-air Aging Properties
Hot-air aging test was carried out by a normal oven-testing method according to JIS K6257. A specimen was maintained at 135xc2x0 C. for 168 hours, and then the dry physical properties were measured by the method mentioned above in (1). The change in % or points of the dry physical properties after the hot-air aging was determined.
(3) Oil Immersion
A specimen was immersed in testing lubricating oil #3 at 135xc2x0 C. for 168 hours, and then the dry physical properties were measured by the method mentioned above in (1) and the volume was measured. The change in % of the volume and dry physical properties after the hot-air aging was determined.
(4) Cold Resistance
Gehman torsion test was conducted according to JIS K6261. Temperature (T10) at which the torsion angle became 10 times of the torsion angle at normal temperature (23xc2x0 C.) was measured. Further, TR10 was measured by TR test according to JIS K6261.
(3) Permanent Set
A crosslinkable rubber composition was cured at a temperature of 160xc2x0 C. under a pressure of 10 MPa for 20 minutes by using a mold having an inner diameter of 30 mm and a ring diameter of 3 mm, and then subjected to second curing at temperature of 150xc2x0 C. for 2 hours to prepare a specimen. Permanent set was measured after the specimen was maintained in a 25% compressed state at a temperature of 150xc2x0 C. for 72 hours according to JIS K6262.
(6) Dynamic Properties and Other Properties
A columnar specimen having a diameter of 17.8xc2x10.1 mm and a height of 25xc2x10.15 mm was cured at a temperature of 160xc2x0 C. for 20 minutes, and then subjected to second curing at temperature of 150xc2x0 C. for 2 hours, Then flexometer test was carried out according to ASTM D623-78 to evaluate the dynamic properties. The flexometer test was carried out by using a Goodrich flexometer wherein a dynamic displacement of 4.45 mm was imposed at a temperature of 100xc2x0 C. and an initial load of 25 pounds (11.34 kg) for 25 minutes. Initial static strain (ISC), initial dynamic strain (IDC), final dynamic strain (FDC), heat build-up (HBU; HBU was expressed by the difference between the temperature of specimen as measured and the environmental temperature [100xc2x0 C.]), and permanent set (PS) were determined.
(7) Number Average Molecular Weight, Molecular Weight Distribution
Number average molecular weight (Wn) and weight average molecular weight (Mw) were measured according to gel permeation chromatography using tetrahydrofuran as solvent and expressed in terms of standard polystyrene. A molecular weight distribution Mw/Mn was calculated from Mn and Mw.
(8) Class Transition Temperature
Extrapolated glass transition-initiating temperature (Tig) and extrapolated glass transition-ending temperature (Teg) were measured by using a heat flux differential scanning calorimeter according to JIS K7121 wherein the rate of temperature elevation was changed from 20xc2x0 C./min to 10xc2x0 C./min to enhance the precision of measurement.